If you use Safari 11, the auto-play feature may be disabled, preventing videos from being played. Whether this feature is disabled or not, can be seen by hovering your mouse over the address bar and looking at what is displayed after 'Auto play':

You can enable this feature by going to Safari's settings and selecting 'Auto-play' under the 'Websites' tab. Here you can allow the TrainTool website to play video's by selecting 'Allow All Auto-Play':